1. Field of the Invention
The present invention relates to a technique of specifying peak positions of an analog signal. More specifically the invention pertains to a technique of accurately specifying peak positions even in an analog signal with varying amplitude.
2. Description of the Related Art
Recently proposed projectors have a resonant circuit including a coil and a capacitor for lighting control of a discharge lamp. In these projectors, the discharge lamp is connected in parallel with the capacitor of the resonant circuit. The frequency of a voltage applied to the resonant circuit is adjusted to a resonant frequency of the resonant circuit. A required voltage for discharge is then applied to the discharge lamp connected to the resonant circuit to light on the discharge lamp.
One example of such projectors is disclosed in Japanese Patent Laid-Open Gazette No. H05-217682.
In the projector having the resonant circuit for lighting control of the discharge lamp, the resonant frequency of the resonant circuit may be varied by wear of a discharge gap in the discharge lamp or by a change of temperature characteristic of the discharge lamp. Application of the voltage having the fixed frequency to the resonant circuit under the condition of the varying resonant frequency causes failed application of the required voltage to the discharge lamp and does not keep the discharge lamp on. In order to keep the discharge lamp on even under the condition of the varying resonant frequency, the projector is demanded to change the frequency of the applied voltage with a variation in resonant frequency.
A proposed projector to meet this demand changes the frequency of the voltage applied to the resonant circuit with a variation in electric current generated in the resonant circuit. The lighting control of this proposed projector is described briefly.
In the projector having the discharge lamp connected to the resonant circuit, as the frequency of the voltage applied to the resonant circuit is gradually increased to the resonant frequency, the discharge lamp starts discharge and high electric current runs through the resonant circuit. The electric current in the resonant circuit increases with an increase in frequency of the applied voltage and reaches the maximum at the resonant frequency.
In this proposed projector, an electric current sensor measures the electric current in the resonant circuit. When the measured electric current reaches or exceeds a preset level, the frequency of the applied voltage is varied to maintain a preset phase difference between the phase of the electric current in the resonant circuit and the phase of the applied voltage.
This regulates the frequency of the applied voltage to keep the electric current at or over the preset level in the resonant circuit even under the condition of the varying resonant frequency. This enables the discharge lamp to be stably kept on.
The projector makes comparison between the phase of the electric current in the resonant circuit and the phase of the applied voltage to detect a phase difference. It is preferable to make the comparison at the positions of respective peak levels (hereafter referred to as ‘peak positions’). Identification of the peak positions, however, has much difficulty, and the phase comparison may be performed according to the following procedure.
FIGS. 8(A) and 8(B) show a process of making comparison between the phase of a voltage applied to a resonant circuit and the phase of electric current in the resonant circuit (resonant circuit electric current) in a prior art projector.
FIG. 8(A) shows the waveform of the voltage applied to the resonant circuit with phase detection points. FIG. 8(B) shows the waveform of the resonant circuit electric current with phase detection points. In FIGS. 8(A) and 8(B), the abscissa denotes the time elapsed (cycles N and (N+1) and part of cycle (N+2)), whereas the ordinate denotes a variation in electric current having the center of the amplitude set at a level ‘0’. The open triangles represent peak positions and the closed triangles represent reference positions detected for phase comparison in place of the peak positions (hereafter referred to as ‘phase detection points’). The voltage waveform and the electric current waveform have both positive and negative peaks. For the simplicity of explanation, the following description regards only the positive peaks.
The phase comparison process in the prior art projector sets a threshold value Th1 of the applied voltage and generates a comparison signal that rises to a high level when the applied voltage reaches or exceeds the preset threshold value Th1 as shown in FIG. 8(A). A rising position of the comparison signal is detected and specified as each phase detection point.
The phase comparison process also sets a threshold value Th2 of the resonant circuit electric current and generates a comparison signal that also rises to a high level when the resonant circuit electric current reaches or exceeds the preset threshold value Th2 as shown in FIG. 8(B). A rising position of the comparison signal is detected and specified as each phase detection point.
The phase comparison process then compares the phase detection points of the applied voltage and the phase detection points of the resonant circuit electric current, instead of the respective peak positions, to detect a phase difference between the applied voltage and the resonant circuit electric current.
The prior art phase comparison process described above, however, has some drawbacks.
With an increase in frequency of the applied voltage to the resonant frequency, the amplitude of the resonant circuit electric current gradually increases as shown in FIG. 8(B). When a time period between a start position (0) and a ½ cycle position (π) in each cycle of the resonant circuit electric current is assumed to be ‘1’, the time length between the peak position and the phase detection point is varied, for example, as ‘0.1’ through ‘0.3’ to ‘0.4’ as shown in FIG. 8(B). Namely the phase detection points of the resonant circuit electric current have varied positional relations to the corresponding peak positions.
When a time period between a start position (0) and a ½ cycle position (π) in each cycle of the applied voltage is assumed to be ‘1’, on the other hand, the time length between the peak position and the phase detection point is fixed to, for example, ‘0.1’ as shown in FIG. 8(A). Namely the phase detection points of the applied voltage have fixed positional relation to the corresponding peak positions.
The phase difference between the applied voltage and the resonant circuit electric current detected by the phase comparison at the respective phase detection points is significantly different from the phase difference detected by the phase comparison at the actual peak positions. The inaccurate detection of the phase difference interferes with precise adjustment of the frequency of the applied voltage and prevents stable lighting of the discharge lamp.
This problem is not characteristic of the phase comparison but also arises in detection of phase detection points as peak positions of an analog signal representing a variation in electric current or voltage in the resonant circuit. The phase detection points have varied positional relations to the actual peak positions. Detection of such phase detection points as the peak positions may accordingly have an error out of a preset allowable range.
Such problems are not restricted to the analog signals in the resonant circuit but may arise in detection of peak positions in any analog signal having varying amplitude.